This invention relates to a laser welding apparatus and more particularly to a laser beam alignment and transport system for use in welding apparatus adapted to weld a sleeve within a tube of a nuclear steam generator.
Various approaches to laser welding of sleeves within tubes of tube type heat exchangers and nuclear steam generators in order to effect repairs of defective tubes are described in the above cross-referenced documents. There are difficulties associated with physically fitting such welding apparatus within the channel or header of nuclear steam generators. There are also difficulties associated with aligning the high power laser with the target or weld site.
FIG. 1 shows a prior art laser welding system illustrated in each of the above cross-referenced documents. For the sake of this discussion only, reference to Hawkins et al. U.S. Pat. No. 4,694,137 will be made. However, it should be understood that the Kasner et al. patent and the Hawkins et al. application contain similar subject matter.
The beam transport system 10 depicted in FIG. 1 directs the beam from a high power laser 12 from a laser beam transmitting means, such as a beam transmitter 14, to a remote laser beam receiving means, such as a beam receiver 16, with no physical connection therebetween. Apparatus associated with the beam transport system 10 is used to align the components to direct the beam along an optimum beam path. Proper laser alignment assures that the high power beam does not improperly impinge on any components of the beam transport system 10 or the steam generator 18 to prevent damage which could thereby result. Initial alignment of the beam transport system utilizes a lower power HeNe laser 20 as a source of visible light in conjunction with low power beam detectors 22 and several video cameras 24 or other visual observation devices.
Once the transmitter 14 and the receiver 16 are aligned, for example as set forth in Hawkins et al., the high power laser 12 is aligned with the transport system using appropriate detectors and the high power laser 12 is then actuated and directed to laser beam transmitter 14 which sends the beam to the remote laser beam receiver 16. The diameter of the high power beam is reduced and recollimated by lens system 26 and redirected towards the welding head 28. The beam is directed at a sleeve 30 within a tube 32 of the steam generator 18. The welding head 28 is rotated during the welding operation to create a leak tight seal between the sleeve 30 and the tube 32.
In accordance with the arrangement illustrated in Hawkins et al. it is necessary to install and align the high power laser 12, the laser transmitter 14 and the laser receiver 16 at each site prior to proper operation of the system. Installation and alignment is often hampered by the fact that equipment around and near the nuclear steam generator 18 restricts access thereto. Also, steam generators at various locations are often different and the setup of the equipment and alignment procedures are often unique to each different type of steam generator model.
In addition to problems associated with individual steam generators, it is also difficult to align the high power laser 12 with the weld site in the sleeve 30. The high power laser 12 is physically located about ten feet or more from the tube or tubes 32 to be repaired.
As is apparent from Hawkins et al. and the other related applications, the transmitter 14 and the receiver 16 have movable wrists 42 which carry beam deflecting mirrors 36. The wrists 42 are driven into selected positions by means of motor and gear arrangements 46 and 48 described in detail in Hawkins et al. Accurate orientation and positioning of the wrists 42 is hampered by the inherent inaccuracy associated with gears. Backlash associated with the gears 48 may be calculated mathematically and refined experimentally so that the wrists 42 may be oriented more accurately. For example, the motors 46 may be stepper motors having many steps per second degree of rotation. However, because alignment of the laser must be precise, even the gear lubrication must be considered. When one or more of the motors 46 is reversed, for example, the lubricants for each of the gears 48 contact before the gear teeth contact. Thus, the wrist does not respond precisely and immediately to the reversal of the gear 48. When a sufficient amount of lubricant is displaced so that the gears 48 firmly contact each other then wrist 42 movement may be more accurately controlled through motor 46 movement. The term soft backlash is used to describe the range of gear travel necessary to displace lubricant between two gears so that the response of the gears may be predictable. The term hard contact refers to the condition of firm contact between gear teeth. Movement required to move from the soft backlash to the hard contact condition is determined and recorded for use during actual alignment processes. However, soft backlash varies depending upon the type of movement initialized by the motor. If a particular motor and gear set 46,48 is operated frequently or for a relatively prolonged period over a particular time interval, the soft backlash will be different than if the same motor and gear set 46,48 is operated once over a short time interval. Another significant factor is whether the motor and gear set 46,48 has been operated recently, i.e., within the previous few minutes. Thus, the soft backlash tends to drift, which makes accurately aligning the laser beam extremely difficult.
It has also been found that the operation of a motor 46 and gear 48 creates a bending moment or deflection of the wrist 42 with which the particular motor and gear is associated. Deflection of the wrist 42 thus creates an additional complication and difficulty in connection with aligning the high power laser 12.